In the field of graphic arts, an image formation system capable of giving a photographic characteristic of superhigh contrast (especially having a gamma value of 10 or more) is required, so as to attain a good reproduction of a dot image of continuous gradation as well as a good reproduction of a line image.
A specific developer called a lith developer has heretofore been used for such a purpose. The lith developer contains only hydroquinone as a developing agent, and uses a sulfite preservative in the form of an adduct of a sulfite with formaldehyde in order to preserve the infectious developability of the lith developer. The concentration of the free sulfite ion in the developer is kept extremely low (usually 0.1 mol/liter or less). Such a lith developer is extremely easily oxidized with air and cannot last for more than 3 days, which is a serious defect.
Methods for obtaining a photographic characteristic of high contrast by the use of a stable developer are disclosed in U.S. Pat. Nos. 4,224,401, 4,168,977, 4,166,742, 4,311,781, 4,272,606, 4,211,857, 4,243,739, etc., where a hydrazine derivative is used. According to the methods, a photographic characteristic of superhigh contrast and high sensitivity may be obtained and, moreover, addition of a sulfite of high concentration to a developer is possible. Accordingly, the stability of the developer against air oxidation is remarkably improved, as opposed to the lith developer. In the methods using a hydrazine compound for forming a superhigh contrast image, however, there are various problems including the variation of the pH value of the processing solution due to processing fatigue or aerial fatigue, the lowering of the concentration of the processing solution due to the depletion of the developing agent or accumulation of an inhibitor, each of which results in lowering of the contrast of the gradation.
Accordingly, means for intensifying the effect of the hydrazine compounds to elevate the contrast of photographic materials are being strongly sought after, and Japanese patent application (OPI) No. 167939/86 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application ") illustrates the use of a phosphonium salt compound, Japanese patent application (OPI) No. 198147/86 illustrates the use of a disulfide compound, and Japanese patent application (OPI) No. 140340/85 illustrates the use of an amine series compound, as a contrast-intensifying agent. Even by the use of these compounds, however, it was still impossible to prevent the eventual lowering of the contrast of photographic materials during the processing thereof.
On the other hand, regarding a bright room-type photographic material with a low sensitivity which contains a hydrazine compound, there is, for example, a silver halide photographic material containing a water-soluble rhodium salt in Japanese patent application (OPI) Nos. 83038/85 and 162246/85. In the material, however, the addition of a rhodium in a sufficient amount so as to lower the sensitivity injures the intensification of the contrast by the action of the hydrazine compound, whereby the desired sufficient high contrast image could not be obtained.
In addition, Japanese patent application (OPI) No. 157633/84 illustrates a method for preparation of a silver halide photographic emulsion which contains a water-soluble rhodium salt in an amount of from 10.sup.-8 to 10.sup.-5 mol per mol of the silver halide and an organic desensitizer where the sum of the cathodic potential and the anodic potential in polarography is positive. However, although the sensitivity may surely be lowered by the method, it is impossible to obtain a sufficient high contrast image which can be utilized in the industrial field of the present invention by the method. Needless to say, Japanese patent application (OPI) No. 157633/84 does not suggest the use of any hydrazine compound.
Hitherto, in a high contrast silver halide photographic material containing a hydrazine compound, the incorporation of an organic desensitizer so as to lower the sensitivity was technically extremely difficult. This is believed to be so because the hydrazine compound has a fundamental function of participating in the development procedure so as to cause the nucleating infectious development because of the electron-donating property thereof to silver halides thereby to give a high contrast image, while the organic desensitizer is a photoelectron acceptor to accept photoelectrons during the image exposure and has a function to lower the sensitivity by interfering with the latent image formation and, on the other hand, the desensitizer also accepts electrons donated from the electron donor such as the hydrazine compound during the development procedure so as to interfere with the nucleating infectious development and further to inhibit the formation of a high contrast image.
Japanese patent application (OPI) No. 62245/81 illustrates a method of forming a high contrast image where a photographic material is developed in the presence of a tetrazolium compound so that the development in the tow part of the characteristic curve is inhibited by the tetrazolium compound. However, this method also has various problems in that the tetrazolium compound-containing silver halide photographic material deteriorates during storage whereupon only a low contrast image can be obtained, that the reaction product from the tetrazolium compound formed by development processing partly remains in the film processed to cause a stain on the film, and that the film often has unevenness of development.
As mentioned above, the conventional method of forming a high contrast image by the use of hydrazine compound is always accompanied by the problems that low contrast images are often obtained in the step of running, i.e., continuous, processing or, when a rhodium salt or an organic desensitizer is added so as to lower the sensitivity of the image, low contrast images are always obtained. In other words, it was extremely difficult to lower the sensitivity of the superhigh contrast image obtained by the use of a hydrazine compound while maintaining the high contrast thereof.
In addition, the hydrazine compound is often added in a large amount so as to intensify the high contrast whereby the strength of the emulsion film is weakened, the storage stability is deteriorated or the excess amount of the hydrazine compound used often dissolves out into the developer during running processing, and thus, the use of such large amount of the hydrazine compound often has a bad influence on the photographic materials to be processed. Accordingly, it is also desired to positively elevate the contrast of photographic materials while using only a small amount of hydrazine compounds.